This invention relates to non-reciprocal couplings for optical fibers, and more particularly, to an optical circulator.
An optical circulator is a non-reciprocal optical device which directs light entering one port to the next higher number port. Thus, light launched into a first port propagates to, and exits through a second port; however, optical signals launched into the second optical port are blocked from reaching the first port and are directed to a third sequential port. Thus, optical circulators, generally, allow signals to travel in only one direction between any two ports.
In a closed circulator, light transmitted into the third (or other highest number port) is passed to the first port. In an open three port circulator, light directed into the third port is extinguished within the circulator without being transmitted to any other active port.
Examples of circulators known in the art are described in U.S. Pat. No. 5,909,310 to Li et al. (Jun. 1, 1999) and U.S. Pat. No. 5,930,039 to Li et al. (Jul. 27, 1999), both of which are incorporated herein by reference.
For certain applications, which employ polarization-dependent devices requiring polarized light in a known polarization state, commercially available known circulators suffer from the disadvantage that the light that emerges from the ports is unpolarized. These applications require the use of additional components aside from the circulator to produce the polarized beams which are required. For example, in C. R. Doerr et al., IEEE Photonics Technology Letters, Volume 12, Number 9, September 2000, incorporated by reference herein, a channelized equalizer is described requiring only one polarization on the chip. The setup requires a polarization beam splitter (PBS) in addition to a circulator for proper functioning.
It would be useful to have the light that exits and enters at least one port of an optical circulator split into two linearly polarized beams, such as along orthogonal or parallel axes. The advantage of such a circulator is that a device which is polarization-dependent can be connected between the two beams at this port so that only light of a desired predetermined polarization passes through the device.
It is an object of the present invention to provide an improved optical circulator.
Thus, in one aspect, the invention provides an optical circulator comprising a plurality of ports, wherein an input port is disposed to launch unpolarized light into the circulator, two sequential ports are disposed to receive polarized light, and an exit port is disposed to receive unpolarized light; a beam-splitting and -combining element, configured to split a beam of light propagating in a forward direction along a first light path, into a second and a third light path, the second and third light paths having mutually-orthogonal polarization, said splitting/combining element also configured to combine mutually-orthogonally-polarized light beams propagating in the reverse direction along a fourth and a fifth light path into a single beam of light propagating along a sixth light path; and a non-reciprocal polarization rotator, configured to rotate polarization directions of light propagating along the second and third light paths and to rotate polarization directions of light propagating along the fourth and fifth light paths.
In another aspect, the invention provides a method for circulating and polarizing light. The method comprises providing a plurality of ports, wherein an input port is disposed to launch unpolarized light into the circulator, two sequential ports are disposed to receive polarized light, and an exit port is disposed to receive unpolarized light; splitting a beam of light propagating in a forward direction along a first light path, into a second and a third light path, the second and third light paths having mutually-orthogonal polarization; non-reciprocally rotating the polarization directions of light propagating along the second and third light paths; combining mutually-orthogonally-polarized light beams propagating in the reverse direction along a fourth and a fifth light path into a single beam of light propagating along a sixth light path; and non-reciprocally rotating polarization directions of light propagating along the fourth and fifth light paths.